Acousto-optic (AO) devices utilize acoustic waves to change the transmission characteristics of an acousto-optic medium (e.g., optical crystals such as fused silica, arsenic trisulfide, tellurium dioxide and other materials that are known in the art). The acoustic waves modulate the refractive index of the acousto-optic medium, with a spatial periodicity equal to the wavelength of the acoustic waves in the acousto-optic medium, and effectively create a diffraction grating that can be tuned by changing the frequency and/or amplitude of the acoustic waves. The acoustic waves are generated with acoustic transducers (i.e., piezoelectric devices) that are driven by radio frequency (RF) tuning signals.
Different types of acousto-optic devices are possible based on different choices of acousto-optic materials, wave geometry and desired performance parameters. AO modulators are used to adjust the power diffracted from a laser beam at a specified diffraction angle and are typically driven with a variable amplitude, fixed frequency RF signal. AO deflectors are used to adjust the angle of diffraction of an incident laser beam and are typically driven with a fixed amplitude, variable frequency RF signal. AO tunable filters (AOTFs) are used to select a specific wavelength of a broadband or multi-wavelength light source and are typically driven with a variable frequency RF signal. Each of these applications requires an adjustable RF signal source. Direct digital synthesis (DDS) allows RF sources to be adjusted programmatically through a digital interface.
Generally, the RF signal frequency needed to achieve a desired diffraction result is a function of many variables, including design variables (e.g., material selection, alignment of acoustic and optical axes, acoustic transducer efficiency, etc.) and environmental variables such as temperature. Closed-loop feedback systems can be used to adjust the RF source to maintain a desired performance parameter (e.g., diffraction angle or diffraction wavelength), but work in only a limited range of applications where the performance of the diffraction can be monitored and require an optical detector and feedback circuitry that adds cost and complexity to an AO system.
Acousto-optic tunable filters generally vary the applied RF signal frequency over a broad range, for example from 80 MHz to 150 MHz, to achieve wide wavelength tuning of the diffracted wavelength. For many applications (e.g., in fluorescence detection), precise wavelength control is needed to optimize system performance. The effect of varying temperature, variations in device manufacturing or alignment, and other factors will change the required frequency from one system to the next even for the same nominal performance parameters. Optimizing performance under these variations requires each device either to be operated under a feedback control system or to be calibrated after installation in the final system.